1. Field of the Invention
The present invention relates to a process for preparing a high-temperature superconducting thin film, more particularly to a process for preparing a high-temperature superconducting thin film possessing improved crystallinity and electrical properties on a substrate.
The high-temperature superconducting thin film means a thin film made of a new type oxide superconductor possessing the critical temperature (Tc) of higher than 30 K. such as Y-Ba-Cu oxide, Bi-Sr-Ca-Cu oxide, Ti-Ba-Ca-Cu oxide or the like.
2. Description of the Related Art
These new type high-temperature superconductors are expected to be utilized in actual uses. In order to fabricate electronics devices such as Josephson element, superconducting transistors or the like, it is indispensable to prepare thin films of these superconductors.
It is known that these high-temperature oxide superconductors show anisotropy in their superconducting properties. In fact, their crystals show higher critical current density (Jc) along a direction which is perpendicular to c-axis than the other directions. Therefore, a variety studies have been made to prepare c-axis oriented thin films of these oxide superconductors in order to flow a larger amount of electric current in parallel with a surface of the thin film, and now the c-axis oriented single crystal thin films of high quality can be prepared by sputtering, vacuum-evaporation, laser abrasion technique or the like on single crystal substrates such as SrTiO.sub.3 and MgO.
These high-temperature superconductors exhibit anisotropy in their coherent length also. Namely, the coherent length along a-axis is longer than those along c-axis. In fact, the coherent length along c-axis is several angstroms (.ANG.) while the coherent length along a-axis is several ten angstroms (10 .ANG.). In certain application of oxide superconductors such as electronics devices, for example Josephson Junctions, the a-axis oriented thin films possessing such longer cherent length are requested. The "a-axis oriented thin film" means thin films made of crystal whose a-axis is substantially perpendicular to a surface of the thin film.
It is known that such a-axis oriented thin films of oxide superconductors can be prepared or realized, for example by sputtering, under such a condition that a substrate on which the thin film is deposited is heated at a temperature which is lower by about 10.degree. C. than a temperature which is requested to realize the c-axis oriented thin film.
However, if the substrate temperature is lowered, the crystallinity of the resulting thin film of oxide superconductor is spoiled because oxygen supply into the crystal become insufficient. Therefore, the a-axis oriented thin films of oxide superconductors prepared by prior arts show relatively poor electrical properties.
In the case of the other technique than sputtering, it is difficult to prepare the a-axis oriented thin films of oxide superconductors even if the substrate temperature is controlled or lowered. Particularly, it is very difficult to prepare the a-axis oriented thin films of oxide superconductors by vacuum-evaporation. The vacuum-evaporation technique have such many merits that a composition in the thin film can be controlled easily and a thin film of large area can be prepared at relatively higher deposition rate. Therefore, there is very strong demand to establish a vacuum-evaporation technique which can prepare the a-axis oriented thin films of oxide superconductors of high quality.
An object of the present invention is to solve the problems and to provide a process for preparing the a-axis oriented thin films of oxide superconductors possessing improved electrical properties on a substrate.
Another object of the present invention is to provide a process which makes possible to prepare such a-axis oriented thin films of oxide superconductors by vacuum-evaporation technique.